Space Station Instrument Finds Evidence of Dark Matter

Thursday, April 4, 2013

The first results from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station were discussed during a Press Briefing at NASA Headquarters. Those results confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays. Antimatter is rare in the universe today. Because Earth receives a limited amount of energetic antimatter, antiparticles serve as unique messengers of high-energy phenomena in the cosmos, or signatures of exotic new physics.

The first results from the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station were discussed during a Press Briefing at NASA Headquarters. Those results confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays. Antimatter is rare in the universe today. Because Earth receives a limited amount of energetic antimatter, antiparticles serve as unique messengers of high-energy phenomena in the cosmos, or signatures of exotic new physics.

These were the first released results of the Alpha Magnetic Spectrometer (AMS) experiment. AMS is mounted on the exterior of the International Space Station and is designed to study cosmic rays before they interact with the Earth’s atmosphere. Cosmic rays are high-energy particles that permeate space, and may hold clues to the existence of dark matter.

Dark matter was postulated by Fritz Zwicky in 1934, to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. According to new supernova observations and Big Bang cosmology, dark matter accounts for 23 percent of the total mass-energy of the observable universe.

The AMS results are based on some 25 billion recorded events, including 400,000 positrons, recorded over the course of one and a half years. Positrons are the antimatter counterpart of the electron. The AMS detector measured the ratio of positrons to the total number of electrons and positrons. This “positron fraction”, should be small, and should fall as energy increases, according to standard astrophysics. Instead, AMS results show that the positron fraction increases from roughly 5% at an energy of 10 GeV to more than 15% at 250 GeV. The data showed no variation over time, nor any preferred incoming direction.

Sam Ting, the Nobel Prize-winning particle physicist (inset), dreamed up AMS and worked tirelessly to make it reality.

The excess of antimatter in the cosmic ray flux was first observed nearly two decades ago, but could not be measured with enough certainty. The AMS data provides the most accurate measurements to date. According to AMS spokesperson Samuel Ting of the Massachusetts Institute of Technology, “AMS is the first experiment to measure to 1% accuracy in space.” The Nobel Prize winning particle physicist is the force behind AMS.

What produced the excess in the positron fraction? A tantalizing possibility is that the positrons arise from dark matter. A theory known as supersymmetry proposes that dark matter consists of weakly interacting massive particles, or WIMPs.

Image Source: Wikipedia commons

Positrons could be produced when two WIMPs collide and annihilate each other, producing an electron-positron pair, with the energy of those particles limited by the mass of the WIMPs. If that is the case, theorists expect that the fraction should increase with energy levels until it reaches a “cutoff” energy where the fraction would begin to fall again. AMS researchers see signs that the positron fraction levels off at 250 GeV, which could mean that the cutoff might be just beyond that energy level.

There may be other possibilities for the cause or causes of the excess positrons. One of the other leading candidates is that pulsars are creating positrons. But Ting remains hopeful, even while acknowledging other possibilities, saying “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”